Increasing size of uv map within blender
As Mr Zak mentioned, the UV map represents points in Texture Space that range from 0.0 to 1.0 and this cannot be changed. However, from your question it appears that you have a UV mapping that relates to an existing image and you want to make some additional space - presumably to add additional faces.

If this is the case then you can resize the image in an external application (for example, in Gimp you could use the Canvas Size option to increase the canvas to 200% in each direction, leaving the original unchanged and some 'blank' space to the right, top and top-right.

When you re-import this image back into Blender the 'islands' obviously no longer match up - since they are still positioned in the original locations in the 0.0 to 1.0 UV texture space but the image is now twice as large (and so the relevant parts of the image are half the size in relating to the mapping).

To address this you simply need to accurately resize the UV mapping (rather than trying to line it up by hand) and you can use the editing tools to achieve this.

Select all faces in the 3D view in Edit mode (Tab). Then in the UV Editor window, select all with A. Open the Properties panel with N and set the Cursor Location to the corner of the UV map relating to where you've positioned your original image (eg, top-left (0.0,1.0), top-right (1.0,1.0), bottom-left (0.0,0.0), bottom-right (1.0,0.0)).

Next set the Rotation/Scaling pivot point (in the toolbar at the bottom of the UV Editor window) to 2d Cursor - this will make the scaling relative to the Cursor Location you have just set.

Next, to accurately rescale your UV map to your 200% rescaled image you need to resize it to 50% - so press S, type '0.5', press Enter.

Since the image was rescaled to 200% and the UV map coordinates are scaled to 50% the UV mapping and the image islands should match up - as they were before rescaling.